Flat panel displays typified by liquid crystal display devices and light-emitting display devices are widely used for displaying images. Although the transistors used in these display devices are mainly manufactured using silicon semiconductors, attention has been drawn to a technique in which a metal oxide exhibiting semiconductor characteristics is used for transistors instead of a silicon semiconductor in recent years. For example, in Patent Documents 1 and 2, a technique is disclosed in which a transistor manufactured using zinc oxide or an In—Ga—Zn-based oxide for a semiconductor layer is used in a pixel of a display device.